Recently, with increasing awareness of environmental protection, the exhausted battery should not be arbitrarily discarded but needs to be recovered because the electrolyte solution of the battery usually contains toxic and corrosive substances. If the exhausted battery is mounted within an electronic device, the weak residual electricity will be slowly consumed. Until the electricity is completely consumed, the electrolyte solution of the battery may flow out of the battery to corrode the electronic circuitry inside the electronic device. Under this instance, most users may discard the corroded electronic device as well as the battery. On the other hand, if the electrolyte solution of the battery is drained out of the electronic device, the electrolyte solution may contaminate the environment. For solving these drawbacks, a commercial electronic device usually has a voltage detecting circuit for detecting the voltage of the battery so as to realize the residual electricity contained in the battery. According to the magnitude of the residual electricity, the voltage detecting circuit issues a corresponding prompt signal to notify the user.
Referring to FIG. 1, a circuit diagram of a conventional voltage detecting circuit is schematically illustrated. The voltage detecting circuit 10 principally comprises a micro controlling unit 100, a boost circuit 101, a power source 102 and an analog/digital converter 103. The micro controlling unit 100 is connected to the boost circuit 101 and the analog/digital converter 103. The power source 102 is also connected to the boost circuit 101 and the analog/digital converter 103. The boost circuit 101 and the power source 102 are both grounded. The analog/digital converter 103 is communicated with the micro controlling unit 100 through a flat cable 108.
Please refer to FIG. 1 again. The power source 102 is a battery. The boost circuit 101 is a DC to DC converter for providing a constant voltage to the micro controlling unit 100. If the voltage inputted into the boost circuit 101 is greater than the constant voltage, the boost circuit 101 is disenabled and the input voltage is directly outputted from the boost circuit 101. Whereas, if the voltage inputted into the boost circuit 101 is smaller than the constant voltage, the constant voltage is outputted from the boost circuit 101 in order to achieve the purpose of boosting voltage. In the voltage detecting circuit 10, the voltage offered by the power source 102, which is smaller than the constant voltage, is transmitted to the boost circuit 101. The constant voltage outputted from the boost circuit 101 is transmitted to the micro controlling unit 100. The constant voltage is defined as a voltage judgment threshold value by the micro controlling unit 100. According to the voltage judgment threshold value, the analog/digital converter 103, which is connected to the micro controlling unit 100, will perform a voltage detecting process.
After the voltage judgment threshold value is defined, the voltage offered by the power source 102 is inputted into the analog/digital converter 103. The analog/digital converter 103 will compare the input voltage with the voltage judgment threshold value and then output a detected voltage. For example, if the analog/digital converter 103 is an 8-bit analog/digital converter and the voltage judgment threshold value is 3 volt, the voltage judgment threshold value (i.e. 3 volt) is divided into 256 (i.e. 2 to the power of 8) digital levels by the analog/digital converter 103. These 256 digital levels include digital values of from 0 to 255. That is, the analog voltage of 3 volt may be indicated as a digital value 255. If the voltage inputted into the analog/digital converter 103 is 1.5 volt, the analog/digital converter 103 will compare this analog input voltage (1.5 volt) with the voltage judgment threshold value (i.e. 3 volt or a digital value 255) and convert this analog input voltage into a digital value 128. Meanwhile, the analog/digital converter 103 discriminates the digital value 128 of the input voltage. After the digital value 128 is converted into the analog voltage of 1.5 volt by the analog/digital converter 103, the voltage detecting process is done.
After the voltage detecting process is done, the messages associated with the detected voltage are transmitted from the analog/digital converter 103 to the micro controlling unit 100 through the flat cable 108. The messages transmitted through the flat cable 108 include for example enabling signals, clock signals and relating data. The flat cable 108 is connected to three connecting ports 104, 105 and 106 of the micro controlling unit 100. The enabling signals, the clock signals and the data are transmitted to the micro controlling unit 100 via the connecting ports 104, 105 and 106, respectively.
The conventional voltage detecting circuit shown in FIG. 1 may be optionally connected to a liquid crystal display during the voltage detecting process. The magnitude of the detected voltage may be shown on the liquid crystal display to be viewed by the user. If the detected voltage drops down to a certain valve, a warming signal is issued by the liquid crystal display to notify the user that the power source needs to be replaced or charged.
As previously described, the conventional voltage detecting circuit uses the analog/digital converter to detect the voltage of the power source. Since the input voltage is discriminated by the analog/digital converter in multi-stages, multi-stage prompt signals may be generated to notify the user. Although the analog/digital converter is effective to discriminate the voltage of the power source, there are still some drawbacks. For example, the analog/digital converter has complicated circuitry and thus costly. In most instances, the powerful function of detecting the input voltage in multi-stages is usually unnecessary for most electronic devices. Generally, the voltage detecting circuit capable of generating three- or four-stage prompt signals is desired because the circuitry space and the cost are reduced.
Therefore, there is a need of providing a voltage detecting circuit to obviate the drawbacks encountered from the prior art.